I do see something of a similarity, though the Voicenorth "V" is 'broken' on the right side below the comma-like serif, and their background color block is circular while Vivaldi's is square. Additionally, Vivaldi's "V" is rounder and softer in appearance. Probably what contributes most to the seeming similarity is their joint use of a white letter on a red background, which at first glance tends to obscure the several differences. The key point, though, is that Voicenorth's business line is totally different from Vivaldi's, so confusion is unlikely to be an issue.

Companies almost never explain their logos, but arguably, for the top-right element Voice has a better use of its quote than Vivaldi’s blobby serif.

Yet the unconscious recognition of their similarity is undeniable, more so than any other Vs that @Ayespy listed. The different nuance is only apparent when comparing them side by side, and regarding the shape difference, Vivaldi even published a circle-square transition:

Opera went thru several logo changes, I expect Vivaldi will do the same.